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The threat actor responsible for large Dridex and Locky distribution campaigns in the past has been using a brand new backdoor in attacks over the past couple of months, Proofpoint reports.
Tracked by Proofpoint as TA505, the adversary has been distributing a new backdoor named ServHelper since November 2018, and also serving the FlawedGrace malware to its victims. The attacks hit banks, retail businesses, and restaurants.
The ServHelper malware, the researchers say, has two variants, one designed to provide remote desktop capabilities to the attackers, and another primarily designed as a downloader. This second variant of the malware was observed serving the FlawedGrace remote access Trojan, Proofpoint’s security researchers report.
The first campaign delivering ServHelper was observed on November 9 and featured only thousands of email messages. Mainly targeting financial institutions, the attack used Microsoft Word or Publisher attachments containing malicious macros that would download and execute the malware.
A second campaign was observed on November 15, featuring tens of thousands of messages and targeting the retail industry, in addition to financial institutions. The campaign featured Microsoft “.doc”, “.pub”, or “.wiz” attachments that attempted to download the downloader version of ServHelper.
A third attack, observed on December 13, targeted retail and financial services customers with Word attachments with embedded malicious macros, PDF attachments with URLs linking to a fake page, and direct URLs to a ServHelper downloader executable. As part of this campaign, the malware also attempted to download and execute the FlawedGrace RAT.
ServHelper is written in Delphi and appears to be under active development, with new commands and functions being added to it with each new campaign.
The backdoor variant of the malware has more features and focuses on setting up reverse SSH tunnels to facilitate access to the compromised host via Remote Desktop Protocol (RDP), which allows the attackers to hijack legitimate user accounts. The downloader variant lacks both the tunneling and hijacking functionality.
ServHelper uses HTTP protocol on port 443 (HTTPS) and, less frequently, port 80 (HTTP), to communicate with the command and control (C&C) server.
The security researchers observed in the malware commands to implement keep-alive functionality, set a reverse SSH tunnel, set a sleep timeout, copy Firefox profile, copy Chrome profiles, kill the SSH tunnel for a particular remote port, get a list of active SSH tunnels, kill all SSH tunnel processes, execute shell command, fetch and run an executable, remove itself from the machine, load DLLs, hijack a user account, and set up an “alerting” mechanism.
The FlawedGrace RAT is written in C++, is very large, makes extensive use of object-oriented and multithreaded programming techniques, and contains support for a multitude of commands. The malware was initially discovered in November 2017, but hasn’t been observed in active campaigns until the recent ServHelper campaigns.
The malware uses a complicated binary protocol for its C&C and can use a configurable port for communications, yet the observed samples have used port 443. Observed communication includes an initial beacon from the infected system, a key verification message from the system, a key exchange message from the C&C server, and a message containing various system and malware information.
“Threat actor TA505 is both consistent and prolific. When the group distributes new malware, it may be a blip (like Bart ransomware, which was only distributed for one day in 2016) or like Locky ransomware it may become the dominant strain of malware in the wild,” Proofpoint concludes.
Related: Phishing Campaign Delivers FlawedAmmyy, RMS RATs
Related: Dridex/Locky Operators Unleash New Malware in Recent Attack
Ionut Arghire is an international correspondent for SecurityWeek.
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